Reverse Linked List in k Groups

Challenge Points: 30

Level: Hard

Given the head of a linked list, reverse the nodes of the list in groups of size k and return the new head.

• k is a positive integer and k ≤ the length of the list.

• If the number of nodes at the end is less than k, leave them as they are.

• Only the node links may be changed, not the node values.

Input: head = [10,20,30,40,50,60], k = 3

Output: [30,20,10,60,50,40]

Explanation:

The first three nodes are reversed, then the next three.

Input: head = [5,6,7,8,9], k = 2

Output: [6,5,8,7,9]

Explanation:

Nodes are reversed in pairs; the last node remains as is.

Input: head = [1], k = 1

Output: [1]

Explanation:

A single node stays the same.

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